The present invention relates to methods for making semiconductor devices, in particular, semiconductor devices that include high-k gate dielectric layers.
MOS field-effect transistors with very thin gate dielectrics made from silicon dioxide may experience unacceptable gate leakage currents. Forming the gate dielectric from certain high-k dielectric materials, in place of silicon dioxide, can reduce gate leakage. Such a dielectric may not, however, be compatible with polysiliconxe2x80x94the preferred material for making the device""s gate electrode. Placing a thin layer of titanium nitride, which is compatible with many high-k gate dielectrics, between a high-k gate dielectric and a polysilicon-based gate electrode may enable such a dielectric to be used with such a gate electrode. Unfortunately, for devices built on certain types of substrates, the presence of such a layer may increase the transistor""s threshold voltage, which is undesirable.
Accordingly, there is a need for an improved process for making a semiconductor device that includes a high-k gate dielectric. There is a need for such a process in which a polysilicon-based gate electrode is formed on such a gate dielectric to create a functional devicexe2x80x94without causing undesirable work function shifts. The method of the present invention provides such a process.